1. Field of the Invention
The present invention relates to liquid crystal display (LCD) devices and more particularly to transflective LCD devices, which use the reflected and transmitted brightness.
2. Description of the Related Art
Reflective LCD devices use the reflected brightness from room light, while transmissive LCD devices use the transmitted brightness from an internal light source, such as a backlight.
The reflective LCD devices, which are used as display devices of portable information terminals, are advantageous over the transmissive LCD devices, in power consumption, thickness and weight. Such advantages derive mainly from the elimination of a backlight. However, the transmissive LCD devices are advantageous in visibility in dark environment.
Commonly, the LCD devices include a layer of liquid crystal (LC) molecules. Examples of modes are a twisted nematic (TN) mode, a single polarizing plate mode, a super twisted nematic (STN) mode, a guest host (GH) mode, a polymer dispersed liquid crystal (PDLC) mode, and a cholesteric mode. A switching element is provided per pixel to drive the liquid crystal layer. A reflector or a backlight is provided within or outside of the LC cell. To produce fine and high visibility in image, the LCD devices employ an active matrix drive system, in which switching elements, such as, thin film transistors (TFTs) or metal/insulator/metal (MIM) diodes, are attached to each pixel to switch one “on” or “off”. In such LCD devices, a reflector or a backlight accompanies the active matrix drive system.
One example of known transflective LCD devices is illustrated in FIGS. 14 and 15. The same transflective LCD device is found in Kubo et al. (U.S. Pat. No. 6,195,140 B1 issued Feb. 27, 2001) and JP 2955277 B2. Kubo et al. illustrates the same structure in FIGS. 1 and 29 and provides a description on FIG. 1 from line 49 of column 8 to line 11 of column 11 and a description on FIG. 29 in lines 53 to 63 of column 27. JP 2955277 B2 illustrates the same structure in FIGS. 1 and 10. Both JP 2955277 B2 and Kubo et al. claim priority based on JP patent application No. 9-201176 filed Jul. 28, 1997.
FIG. 14 is a plan view of one pixel portion of an active matrix substrate, illustrating gate lines 4 and source lines 5 that are disposed along the peripheries of pixel electrodes 3 and cross each other at right angles. TFTs 6 are formed in the vicinity of the respective crossings of the gate and source lines 4 and 5. A gate electrode and a source electrode of each TFT 6 are connected to the corresponding one gate line 4 and the corresponding one source line 5, respectively. Each of the pixel electrodes 3 includes a reflective region 7 formed of a metal film and a transmissive region 8 formed of indium/tin oxide (ITO).
In the reflective mode, the room light passes through the LC layer to the reflective regions 7 of the pixel electrodes 3. At the reflective regions 7, this light is reflected and returns through the LC layer to a viewer. In the transmissive mode, the light from a backlight passes though the transmissive regions 8 of the pixel electrodes 3 and the LC layer to the viewer.
In the reflective regions 7, the room light and the returned light pass through the LC layer in the opposite directions before reaching the viewer. In the transmissive regions 8, the light from the backlight passes through the LC layer once before reaching the viewer. When both reflective and transmissive modes are used simultaneously, the difference in optical path makes it difficult to optimize the output, such as brightness and contrast. One approach to this problem is found in the structure shown in FIG. 15. According to this structure, the thickness of the LC layer dr in the reflective regions 7 is different from the thickness of the LC layer df in the transmissive region 8. In the reflective regions 7, the thickness dr is adjusted by adjusting the thickness of an insulating layer 17, which lies between a transmissive electrode 19 and on a reflective electrode 1. In FIG. 15, the reference numeral 25 indicates a counter electrode.
To eliminate the difference in optical path, the setting is such that the ratio between the thickness dr and the thickness df is about 1:2. This requires that the insulating layer 17 be thick almost as much as half the thickness of the LC layer. Thus, in the reflective regions 7, the insulating layer 17 with several micron meters thick is required, resulting in an increased fabrication processes. Besides, the provision of such insulating layer 17 in each reflective region 7 prevents the overlying transmissive electrode 19 from having surface flatness. The surface of the transmissive electrode 19 is coated with material to form an alignment film. The surface of the alignment film is not flat. With such alignment film, robbing method may not provide a degree of alignment of LC molecules as high as expected.
An object of the present invention is to provide a liquid crystal device in which the difference in optical path between the reflective mode and transmissive mode is reduced with the plane surface of a substrate kept.